1. Field of the Invention
The present invention relates in general to drive units of a type including an alternating current motor (which will be referred to a.c. motor hereinafter) and more particularly to a so-called a.c. motor-inverter integrated drive unit in which an a.c. motor and an inverter are integrally combined.
2. Description of Related Art
One of the above-mentioned integrated drive units is shown in Japanese Laid-Open Patent Application (Tokkai) 2000-166176. In this unit, elements of the inverter are concentrated on an upper portion of the a.c. motor. However, this arrangement tends to cause increase in length between output terminals of power drivers of the inverter and input terminals of stator coils of the a.c. motor. Thus, connection of each output terminal of the inverter to corresponding input terminal of the a.c. motor needs an elongate wiring member, which brings about a considerable heat loss caused by an increased electric resistance possessed by the wiring member. Besides the heat loss, the elongate wiring member causes generation of various noises.
In the integrated drive unit of the above-mentioned published application, cooling passages are respectively formed in a cylindrical stator structure of the a.c. motor and a power driver part of the inverter, and these cooling passages are connected through a generally U-shaped pipe which is exposed to the outside of the unit. That is, the a.c. motor and the inverter have each a cooling passage. However, due to its inherent construction, employment of the cooling passage in each of the motor and the inverter causes a bulky arrangement of a cooling system. In particular, when the motor is locked or forced to run at a very low speed under generation of a maximum torque, the cooling system has to bear the sum of the maximum loss of the inverter and the maximum loss of the stator coils of the motor. This means that the cooling system needs a larger or bulky size.